"whosever" is a rare formal word, practically unused in modern English. I don't recommend it. You can use "whoever's" instead, or recast the sentence in some other way.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
"whosever" is a rare formal word, practically unused in modern English. I don't recommend it. You can use "whoever's" instead, or recast the sentence in some other way. "Whose phone is ringing will be ..." is not correct.
The use of "forfeited" is also wrong. The phone would be forfeited by the pupil, not by the teacher. The word you want here is "confiscated".
This is the closest match to a natural English sentence. Still, it is quite awkward.
The person whose phone is ringing must forfeit it to the homeroom teacher.